One of the most frustrating facts about bullying is this: In the vast majority of cases, it takes place in front of an audience of other kids — 88 percent of the time, according to one study. And yet kids who are bystanders intervene only 20 percent of the time. When they do step forward, however, they stop half the bullying they try to head off.

Bystanders, then, represent a major opportunity: Convert more of them into defenders or allies of the target of bullying, and you could take the sting out of one of childhood's enduring harms.

Except that it's not so easy. Adults constantly exhort kids to stand up for kids who are the victims of taunting and cruelty. It's what many of us want to see from our own children — I know I expect my own sons to stand up for the kid getting picked on. But stepping into the middle of a conflict to confront an aggressor is usually asking a lot. "As bullies are often perceived as popular and powerful, it takes a lot to thwart their behavior," as the Finnish psychologist Christina Salmivalli, a leader in the field, puts it in a 2010 paper. In a new study from Harvard, based on in-depth interviews with 23 middle-schoolers, every single one said they supported the idea of being an "upstander" rather than a passive bystander, but "half of them acknowledged that in practice they often laugh when they see others victimizing a peer in school," as the authors put it. Kids want to help, and know they should, but they don't always do it.

It's an example of the classic bystander problem that also affects adults: In a famous study in the late 1960s, researchers showed that people who watch a dangerous situation unfold take longer to help if there are many fellow onlookers. The presence of multiple bystanders diffuses responsibility and also makes people unsure of social cues — they hesitate to be the first one to make a move if no one else has. These findings help explain how Kitty Genovese was brutally and infamously murdered by a knife-wielding attacker in New York in 1964, even though she cried out for help to neighbors in a nearby apartment building. The facts of the story aren't quite as awful as the original telling: Someone did initially scare off the killer by opening a window to yell, "Let that girl alone!" And it may be that none of the neighbors realized that he actually came back. Still, it did take a half-hour for someone to finally call the police — for Genovese, too late.

One of the experts I've come to respect in writing my book on bullying is a former guidance counselor and researcher, Stan Davis, who points out that kids can also help the targets of bullying in quieter, after-the-fact ways, by asking them if they're OK or sending a sympathetic text. Having even one single defender reduces the negative effects for the victim — it makes the bullying less upsetting.

What do we know about kids who come to the aid of the targets of bullies — and how could we expand their number? Salmivalli points out that not surprisingly, kids are more likely to stick up for kids they're friends with. Kids are also more likely to be upstanders if they're in a social circle of friends in which other kids don't bully. The few kids in the Harvard study who stood up for victims of bullying outside their friend group had an objective sense of moral responsibility, "a common commitment to address issues of unfairness," as the researchers put it. They were also "very powerful members of the peer group" — kids with status.

But we can't only rely on the kids who happen to be both moral and high in status. The key is to figure out how the environment of a classroom — or a whole school — can add to their ranks. There is plenty of evidence that aggression and meanness among students can pay off in terms of popularity — it's often a route to moving up the social ladder. So how can a school, or a community, turn that around to reward a kid's willingness to put themselves on the line to stop cruelty instead? It's a question at the heart of every good bullying prevention effort, and one that explains why the programs with success encompass the whole school. Salmivalli focuses on one with particular promise: a program in Finland, called KiVa, that has generated a lot of in international interest based on its record of reducing bullying and increasing peer support for victims. She emphasizes the importance of showing kids that bullying isn't the norm — that most people don't do it. When very few kids step up to challenge bullying, on the other hand, "children may infer that the others think bullying is OK." In the United States, bullying prevention programs like Olweus and Steps to Respect try to encourage more upstanding. But no one here has completely solved the riddle. One person who is trying is Nancy Willard, a longtime advocate in the field who directs Embrace Civility in the Digital Age. Willard has designed a new program with a couple of features I haven't seen before that seem promising, like encouraging teachers and other staff to watch for students who act as allies, not to make them an object of public praise, since that could make them uncool and backfire, but so that the principal can send a letter of thanks home. It's a small idea, but that's somehow how larger cultural shifts start.

—- Bazelon is a Slate senior editor and writes about law, family, and kids. Her new book is "Sticks and Stones: Defeating the Culture of Bullying and Rediscovering the Power of Empathy and Character." Find her at emilybazelon@gmail.com or on Facebook or Twitter.